Technique for forming resin-impregnated fiberglass sheets using multiple resins

ABSTRACT

A method and resultant article are provided which optimize the adhesion of resin to the glass fibers in fiberglass cloth impregnated with a resin and also optimize the adhesion of the impregnated resin to metal sheets laminated to the resin-impregnated cloth. The fiberglass is treated in two or more passes. On the first pass, the fiberglass is impregnated with a first resin which is optimized for adherence to glass fibers and the coated resin is partially cured. In a last pass, the fiberglass is impregnated with a second resin, which is different from said first resin, and is optimized for bonding to metal. The second resin is then partially cured. The first and second resins are selected such that they form a bond with each other when cured.

RELATED APPLICATIONS

Application Ser. No. 08/716,815, filed Sep. 10, 1996, for "Technique forForming Resin-Impregnated Fiberglass Sheets" (Atty. Docket No.EN9-96-028).

Application Ser. No. 08/716,815, filed Sep. 10, 1996, for "Technique forForming Resin-Impregnated Fiberglass Sheets with Improved Resistance toPinholing" (Atty. Docket No. EN9-96-029).

FIELD OF THE INVENTION

This invention relates generally to forming of resin-impregnated cloth,and more particularly to forming improved resin-impregnated fiberglasssheets and the resultant sheets which are specially adapted for use informing chip carriers and the like, and which use resins which haveimproved adhesion to both glass fibers and metals in a laminatestructure.

BACKGROUND OF THE INVENTION

Resin-impregnated fiberglass sheets are commonly used in the formationof printed circuit boards. The fiberglass cloth is typically impregnatedwith the selected thermoset resin which is then partially cured and theimpregnated cloth sheared to form what are known as sticker or prepregsheets. In order to enhance the adhesion of the resin to the fiberglass,often a coupling agent, such as a silane, is coated onto the surface ofthe fiberglass prior to impregnation. The prepreg sheets are then laidup with sheets of metal such as copper or copper-invar-copper (CIC) andlaminated with heat and pressure to fully cure the laid-up laminate withthe metal sheets defining ground, power and signal planes. One of thedesirable characteristics of the resin-impregnated fiberglass sheets isthat the resin-impregnation must cover the fibers of the fiberglass andmust be able to be partially cured to a non-tacky state wherein thesheets can be handled for the lamination process. This is often referredto as a B-stage, a cure state which allows the sheets to be sufficientlyselfsupporting to be laid up as a laminate, but not advanced enough inthe state of cure that they are rigid or nonflowable when heated, andthey can be further cured to a final cure with heat and pressure to forma laminate structure as is well known in the art. As indicated above,this lamination process normally includes the lamination of one or moresheets of metal, such as copper, CIC or other metal, to providenecessary ground planes, power planes and signal planes buried withinthe laminated circuit board. Also in conventional practice, openings areformed, either by drilling or other means, through the fully curedlaminates which form the openings for vias or plated through holes wherethe connections can be made from one surface of the circuit board to theother and to the various internal planes within the laminate asrequired.

A conventional technique of forming the resin-impregnated fiberglasssheets is to provide a coil of the fiberglass material and unwind thefiberglass material from the coil and continuously pass it through atank containing the solution of the desired resin in a solvent, and thenpass the coated or impregnated material through a treater tower whereinheat is applied to drive off the solvent and to partially cure the resinmaterial by initiating crosslinking and then coiling the partially-curedor B staged material into a coil. Thereafter, the partially-curedmaterial is uncoiled and cut into sheets of the desired length. Thesesheets, known as prepreg sheets, are then used in the lamination processdescribed above.

This prepreg material has long been used for manufacturing circuitboards, however, more recently, the same prepreg material and samelaminating techniques that have been used to form a circuit board havebeen used to form chip carriers. A chip carrier is basically asmall-size version of a circuit board where the metallurgy and the layout can be much finer than on a circuit board. Printed circuit boardreliability tests are defined, e.g., in IPC specifications, whereas chipcarrier tests are defined by JEDEC specifications which are derived forceramic carriers and are more severe tests. In addition, because of thefiner geometry of the metallurgy and the lay-out, chip carriers are moreprone to failure from various failure mechanisms.

In forming laminate structures which include resin impregnated glassfibers and metal sheets, such as copper, in the laminate, it is knownthat certain resins adhere better to the glass fibers, and certainresins adhere better to the metal. Thus, there has been a tradeoff as towhether to select a resin for good adhesion to the glass fibers tominimize defects that can occur because of poor adhesion to glassfibers, or to select a resin which has better adhesion to the metal andthus reduce the defects of a laminate structure caused by delaminationof the metal from the resin-impregnated fiberglass. Indeed, in someinstances, a compromise is attempted by mixing two resins, one havingbetter adhesion to glass fibers and the other having better adhesion tometal. This results in a compromise in that neither the adhesion to theglass fiber nor the adhesion to the metal is optimized; and, moreover,this can cause problems by requiring the selection of resins that arecompatible with each other and which can be mixed and will coatproperly. Even when these problems are overcome, there is still theproblem that neither the adhesion to the fiberglass nor the adhesion tothe metal is optimized.

Therefore, it is a principal object of the present invention to providea resin configuration that optimizes adhesion to both the glass fibersand the metal in a laminate.

SUMMARY OF THE INVENTION

According to the present invention, a method and resultant article areprovided which optimize the adhesion of resin to the glass fibers infiberglass cloth impregnated with a resin and also optimize the adhesionof the impregnated resin to metal sheets laminated to theresin-impregnated cloth.

The fiberglass is treated in two or more passes. On the first pass, thefiberglass is impregnated with a first resin which is optimized foradherence to glass fibers and the coated resin is partially cured. In alast pass, the fiberglass is impregnated with a second resin, which isdifferent from said first resin, and is optimized for bonding to metal.The second resin is then partially cured. The first and second resinsare selected such that they form a bond with each other when cured.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a somewhat schematic representation of an apparatus forimpregnating fiberglass cloth with resin and partially curing theimpregnated fiberglass;

FIG. 2A is a very schematic representation of sheets of partially-curedimpregnated resin and copper sheets positioned to form a laminate corestructure;

FIG. 2B is a schematic representation view of a portion of a raw coreformed from sticker sheets and copper sheets;

FIG. 2C is a schematic representation of a raw core and sticker sheetsand copper sheets positioned to be laminated and form a chip carrier;

FIG. 2D is a schematic representation of a chip carrier;

FIG. 3 is a view on an enlarged scale of the portion of woven fiberglasscloth before coating according to this invention;

FIG. 4 is a sectional view taken substantially along the planedesignated by the line 4--4 of FIG. 3;

FIG. 5 is a view similar to FIG. 3 after the initial coating of thefiberglass cloth with a resin and partially cured according to thisinvention;

FIG. 6 is a sectional view taken substantially along the plane designedby line 6--6 of FIG. 5;

FIG. 7 ia a view similar to FIGS. 3 and 5 after a second coating ofresin has been placed over the first coating of the fiberglass cloth;and

FIG. 8 is a sectional view taken substantially along the planedesignated by the line 8--8 of FIG. 7.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

Referring now to the drawings, and for the present to FIG. 1, a somewhatschematic representation of coating of the fiberglass cloth withresin/solvent solution and partially curing the resin is shown. A rollof fiberglass cloth 10 is shown which passes over a guide roll 12 into asolution 14 of the desired resin maintained in a selected solvent. (Itis to be understood that the term "solution" is used here to describethe resin in a solvent, whether or not it be a true solution.) Thesolution 14 is maintained in a tank 16, and the fiberglass cloth fromthe roll 10 is maintained submerged in the tank by a pair of sink rolls18. The fiberglass from the roll of fiberglass cloth 10 is passedthrough the solution 14 into a treater tower 20. The solutionimpregnates the fiberglass cloth 10 with the resin/solvent solution. Theamount of resin on the cloth is usually controlled by metering rolls 19.

The treater tower 20 has a series of rolls 24 over which the fiberglasscloth 10 is reeved, and the treater tower 20 is heated, either byconvection or by radiation, such as infrared radiation or by microwave(not shown), or by some other mechanism to drive the solvent from thesolution and to partially cure the resin by initiating thecross-linking. The tower 20 is divided into several zones in which thetemperature and the gas flow can be controlled independently. Theexiting material, known as prepreg, now is comprised of glass cloth andpartially reacted resin and is largely free of solvent. This state ofthe material is known as a B-stage, and the amount of curing requiredfor the B-stage is well known in the art. Specifically, this B-stagedresin is sufficiently cured to form a sheet of material which has theresin in such a form that it will remain in the fiberglass material andis sufficiently uncured that it can bond to other similar sheets ofmaterial and to metal sheets and be further cured to a hardened stateupon application of heat and pressure. The amount of resin applied tothe sheet is a function of the nature of the fiberglass cloth 10, aswell as the speed of the travel of the fiberglass cloth 10, setting ofthe metering rolls 19, the composition of the solution of resin andsolvent 14, and various physical parameters such as the temperature,viscosity and other well known factors of the solution. Likewise, theamount of cure is a function of the temperature of the tower, the amountof solution and resin applied to the fiberglass cloth, the compositionof the solution and the composition of the resin, all of which also arewell known in the art. The fiberglass cloth 10 as it emerges from thetreater tower 20 in the B-stage state is wound into a coil ofpartially-cured material 26.

In conventional practice, a single pass of the fiberglass cloth 10through the solution 14 and the treater tower 20 is used. Thepartially-cured fiberglass is then ready to be uncoiled and cut intosheets often referred to as prepreg sheets for forming a laminatedstructure.

In order to form a chip carrier or a circuit board, alternate layers ofB-stage cured prepreg sheets and sheets of conductive material such ascopper or copper-invar-copper (CIC) as shown in FIG. 2A are laminatedtogether. In one typical lamination process, one or more, andconventionally two, sheets of prepreg 30a, 30b are laminated between twosheets of copper or CIC 31a, 31b using heat and pressure to form a rawcore 32 (FIG. 2B). Because of the heat and pressure, the two sheets ofprepreg 30a, 30b bond together to form an essentially continuous orsingle layer, and bond to the two sheets of CIC 31a and 31b. The sheetsof prepreg 30a, 30b are essentially fully cured as shown in FIG. 2B witha dotted line indicating their interface.

This raw core can, in the simplest embodiment, be circuitized to form achip carrier or circuit board. Using photolithographic and drillingtechniques, the sheets 31a and 31b are circuitized to form the desiredcircuitry on both sides of the prepreg sheets 30a and 30b and platedthrough holes formed where needed.

In other embodiments, the core 32 is used to form buried planes such asground planes and power planes, or even signal planes. As shown in FIG.2C, two additional sheets of prepreg 33a and 33b are laminated to sheet31a and to a sheet of copper 34a and two additional sheets of prepreg33c and 33d are laminated to sheet 31b and sheet of copper 34b andessentially fully. cured by heat and pressure. These sheets, ready forlamination, are shown in FIG. 2C. Copper sheets 34a and 34b are used toform circuit traces 35a and 35b by subtractive etching. Plated throughholes 35c are formed in a conventional manner as shown in FIG. 2D. Theseraw cores and completed circuit boards or chip carriers are well knownin the art as are the various techniques for forming them, and thus theyare only briefly described.

As described above, when a resin is chosen for impregnation, it iseither optimized for adhesion to glass fibers or optimized for adhesionto metal, or, in some instances as indicated, mixtures of resins can beused which will provide a compromise in which the resin is not optimizedfor either but is somewhere in between.

According to the present invention, a resin system is provided whereinresins are applied in a multiple pass process which in its simplest formis a two pass process. in the first pass, the fiberglass is impregnatedwith a resin which is optimized for adhesion to glass fibers andpartially cured. The amount of resin impregnated is less than the totalloading that is to be the final desired amount. This is shownschematically in FIGS. 3-6.

FIGS. 3 and 4 show a small section of woven fiberglass cloth beforecoating. The cloth has woof fibers 36 and warp fibers 38 which are wovenin a conventional manner to form interstices or windows 40. The glassmay have adhesion-promoting coating such as a silane (not shown) coatedthereon prior to impregnation. After the first pass and partial cure,the resin material forms a coating 42 of partially-cured resin over thefibers 36 and 38 and, in some instances, bridging over the intersticesor windows 40. It is not critical to have significant and substantialcoverage of all the windows during this first pass, although substantialcoverage of the windows may occur. The only critical requirement is thatthe glass fibers be covered with resin during the first pass which isoptimized for adhesion to the fiberglass. Resins containing bismaleimidetriazine (BT) are well suited for adhesion to glass fibers. Significantpin holes 43, as shown in FIGS. 5 and 6, can be present withoutdetrimental effect. The resin must be sufficiently cured during thefirst pass that it will not redissolve in the resin solution of thesecond pass.

The fiberglass, coated and partially cured in the first pass, is left incoil form and is then given a second pass through a solution 14 of aresin. However, the resin in the second solution is selected to optimizeadhesion to metal. Of course, the resin of the second solution must alsoadhere to and bond with the partially cured resin coating 42 and notcause it to dissolve in the solvent/resin solution of the second pass.The resin solution during the second pass coats over the entire coating42 to form an outer or second coating 44. Since the fiberglass cloth onthe second pass also passes through the treater tower 20, the outercoating is partially cured to a B-stage cure, while the heat advancesthe cure of the inner coating 42, but which coating 42 is not fullycured. The second coating 44 forms a bond with the coating 42 during thepartial cure. This condition is shown in FIGS. 7 and 8. The interfacebetween the coating 42 and coating 44 must be a strong bond to preventdelamination and, by the proper selection of resins, this strong bondingcan be accomplished. Epoxy resins provide a good interface to copper andother metals, and therefore an epoxy resin is a good selection for thesecond coating.

It should be understood that the second coating procedure could beperformed on a second coating line in a continuous manner following theemergence of the partially cured cloth from the treater tower 20 afterthe first coating, thus obviating the necessity of coiling and recoatingin the same line.

After the second coating process wherein the outer coating 44 is appliedand partially cured, the sheets are then cut and are laminated in aconventional manner as previously described for raw cores and/or chipcarriers and/or circuit boards. Further heating during the laminationprocess finishes the bonding process of the outer coating 44 to theinner coating 42, making a strong adherent structure.

The following Table I is an example of a procedure and parameters usingparticular resin systems in each pass to form a roll of epoxy fiberglassimpregnated utilizing two passes with two different resins according tothe present invention.

                  TABLE                                                           ______________________________________                                        GRADE 1080 FIBERGLASS CLOTH.sup.(1)                                                        Single Pass                                                                           Two Passes                                                            Prior Art                                                                             First Pass                                                                              Second Pass                                    ______________________________________                                        Treated Weight.sup.(2)                                                                       9.9       5.5       9.9                                        Resin Content  59.6.sup.(3a)                                                                           27.0.sup.(3b)                                                                           59.6.sup.(3c)                              Flow.sup.(4)   25-30     N/A       20-25                                      % Fully Cured.sup.(5)                                                                        25-45     60-70     mixed                                      Line Speed.sup.(6)                                                                            9-13     2-4       9-13                                       M/Min.                                                                        Resin SpG..sup.(7)                                                                           1.1       1.1       1.1                                        Gr/CC                                                                         Resin Solids.sup.(8)                                                                         65%       65%       65%                                        Catalyst Level.sup.(9) (PHR)                                                                 0.13      0.03      0.13                                       Meter Roll Gap..sup.(10) (um)                                                                180-230   180-230   180-230                                    Meter Roll     2.5       2.5       2.5                                        Speed.sup.(11) (M/Min)                                                        Oven temps.sup.(12) :                                                         Zone #1 (°C.)                                                                         110       110       110                                        Zone #2 (°C.)                                                                         115       145       115                                        Zone #3 (°C.)                                                                         160       165       160                                        Zone #4 (°C.)                                                                         175       175       175                                        Zone #5 (°C.)                                                                         175       175       175                                        Zone #6 (°C.)                                                                         160       175       160                                        Air Velocity.sup.(13)                                                                        6         6         6                                          (M/min.)                                                                      Residence Time.sup.(14)                                                                      2.5       11.0      2.5                                        (min.)                                                                        ______________________________________                                        .sup.(1) 1080 is a standard industry grade description of glass fiber         cloth having a given fiber size and weave and has a weight                    of 1.45 gram/yd..sup.2                                                        .sup.(2) The weight of 128 square inch piece of cloth after resin             impregnation.                                                                 .sup.(3a) The percent of resin in the treated cloth. The preferred resin      is Ciba-Geigy's XU8213, which is contained in a solvent which is              preferably methyl ethyl ketone.                                               .sup.(3b) First pass resin - BT epoxy - Varnish formulation:                  Difunctional epoxy (Ciba 9302)                                                                     47.7%                                                    BT blend (Mitsubishi 2060)                                                                         40.2%                                                    Solvent (Methyl Ethyl Ketone)                                                                      7.3%                                                     Yellow epoxy (Shell 1030)                                                                          4.7%                                                     Catylist (PHR) (Zinc Octanoate)                                                                    .03%                                                     .sup.(3c) Second pass resin - Ciba-Geigy high Tg epoxy - formulation:         Multifunctional epoxy (Ciba 8213)                                                                  90.3%                                                    Solvent (Methyl Ethyl Ketone)                                                                      9.7%                                                     Catylist (PHR) (2-Methyl Imidazole)                                                                .13%                                                     .sup.(4) A flow is measured by cutting eight 4" × 4" pieces of          cloth after                                                                   curing, and pressing them together. The amount of resin pressed out is        expressed as a percent of the total resin content.                            .sup.(5) The percent fully cured of the resin as measured by flaking          pieces                                                                        of resin off of the cured cloth and determining the percent cure. In          the case of the percent cured after the second pass, since there is           a mixed cure of the first and second resin, this is an estimate.              .sup.(6) The speed of the cloth as it is driven through the resin             solution                                                                      and the treater tower.                                                        .sup.(7) Resin specific gravity.                                              .sup.(8) Resin solids is the percent of resin in the solvent. The             preferred                                                                     solvent is given.                                                             .sup.(9) The level of the catalyst, which in the preferred embodiment is      2-methylimidazol in parts per 100 parts of resin.                             .sup.(10) The gap between the meter rolls.                                    .sup.(11) The meter roll speed in meters per minute.                          .sup.(12) The oven temperature of the various zones in the treater            tower.                                                                        .sup.(13) The velocity of the air flowing in the treater tower.               .sup.(14) The time the impregnated cloth is in the treater tower.         

As can be seen from the table above, many of the parameters are the samefor both the first pass and the second pass in the present invention.Also, many of the parameters are the same for the second pass of thepresent invention and the prior art single pass. The main differencesare in the line speed and the temperatures of the oven zones on thefirst pass as compared to the prior art single pass technique. The linespeed is significantly slowed down, and the oven temperatures changed insome of the zones. The slowing down of the line speed results in lessresin being impregnated into the cloth than at a faster line speed. Thisis because during the slow line speed on the vertical run of the clothas it is coming out of the resin solution, it has more time to drainand, thus, more of the resin which has been coated onto it drains offback into the solution 14 of the resin. The temperature changes in theoven, coupled with the slower speed results in a more complete cure ofthe first pass resin than is achieved with the prior art single passtechnique; i.e., in the prior art single pass technique, the cure forB-stage is 20-45%, whereas after the first pass of the two passtechnique according to the present invention, there is about a 60%complete cure. This additional cure is desired to insure that, duringthe second pass of the present invention, the resin deposited on thefirst pass and partially cured does not redissolve, but rather remainsas a base on which the second pass resin is deposited. Thus, the slowerspeed and higher temperatures in certain zones of the oven result in amore fully cured resin, coating essentially the fibers and significantamount of the interstices which are then covered and fully filled induring the second pass wherein the parameters are preferably about thesame as the parameters for a single pass technique. Other modificationssuch as change in viscosity, change in percentage of resin, etc., can beused to vary the coating weight.

It should be understood that the particular table of parameters ismerely illustrative and that these can be varied significantly to obtainthe desired results. For example, other cloth styles could be used.Typically, these cloths can include Grade 106, 2116 or 7628, as well asothers which are suitable for resin impregnation. In addition, othercloths based on quartz, s-glass or organic fibers can be used. Withthese other types of cloth, the parameters or conditions of coating onthe two passes are modified to obtain the desired results. Also, otherresins can be used, as was explained above, which would cause theconditions to be changed, all of which is well-known in the art.

Other parameters offering faster through-put are possible, includingusing resins with lower solids contents, higher catalyst levels andhigher oven temperatures. The use of lower solids is advantageous forthe first pass coating as the lower solids help to improve fiber wetout.

The concept of two pass impregnation using different resin chemistriesis incorporated to address a number of different problems depending onthe composition and properties of the various applied resins.

The first pass coating component, which can be impregnated with heat-and/or light-curable resins, provides well-balanced properties, such asheat dimensional stability, heat or soldering resistance, moistureresistance, electrical insulating properties and mechanical propertieswhich are particularly useful for the construction of chip carriers andcircuit boards.

In an exemplary fashion, bismaleimide-triazine (BT), as well asbismaleimide-triazine epoxy blends provide superior adhesion to glassand good resistance to conductive filament formation. These propertiesare important to the long term reliability of circuit boards, as well aschip carriers. In a similar way, the high glass transition temperaturecyanate esters, in combination with other lower glass transitiontemperature cyanate esters, as well as epoxy resins normally used tomodify their properties, applied onto the glass cloth as the firstcoating, provide superior insulation resistance to wet thermalrepetitive exposure, improved adhesion to the glass and good protectionto the glass resin interface. In addition, superior retention ofstrength is believed to be achieved at high temperatures and highmoisture environments. By using such resin combinations, it is believedthat the glass to resin interface remains intact and maintains itsinsulative properties upon various modes of thermocycling.

Subsequent to the first pass, the second pass impregnation isaccomplished by utilizing specially tailored epoxy based formulationsthat provide compatibility to the first pass impregnation materials,good electrical characteristics, resistance to moisture and also provideexceptional adhesion to copper or other metals. The second pass coatingof epoxy constitutes the majority of the volume of the composite, thusthe composite material exhibits much better drilling properties thanwould a pure bismaleimide-triazine-epoxy glass cloth laminate. A costreduction is also achieved from such constructions. An additionalbenefit of the subject invention is that when the first passimpregnation material is overcoated with the hydrophobic in nature epoxybased composition, for example (Araldite 8213), moisture induced defectssuch as "measles" and delamination can be prevented.

Table II below describes -various resins and combinations of resinswhich can be used for the first pass and second pass coatings. Thislisting is not exhaustive, but only illustrative, and other resins andcombinations can also be used. These, however, are the preferred resinsand combinations.

                                      TABLE II                                    __________________________________________________________________________    First pass resin                                                              compositions A  B  C  D  E  F  G  H  I  J  K                                  __________________________________________________________________________    Bismaleimide-Triazine,                                                                     40.2                                                                             53.7                                                                             90.3  40.2                                                                             37.6                                              BT-2060B (70% solids)                                                         Bismaleimide-Triazine,                                                                              90.7                                                    BT-2110 (70% solids)                                                          Difunctional epoxy,                                                                        47.7                                                                             36.6     37.6                                                                             39.7        53.7                                                                             37.3                               Araldite 8011 (72% solids)                                                    Diglycidyl ether of         13.3                                              bisphenol A, Epon 828                                                         Cresol Novolac,             10.1                                              ECN-1280 or ECN-199                                                           Tetrafunctional epoxy                                                                      4.7         4.7               4.7                                (Shell 1031 - 80% solids)                                                     Cyanate Ester, Arocy F-40S        40.5                                        (75% solids)                                                                  Cyanate Ester, Arocy B-40S     90.3                                                                             49.8                                                                             32.5                                                                             36.6                                                                             48.3                               (75% solids)                                                                  Cyanate Ester, REX-379               57.8                                     (72% solids)                                                                  Solvent, Methyl Ethyl                                                                      7.3                                                                              9.7                                                                              9.7                                                                              9.3                                                                              7.3                                                                              9.4                                                                              9.7                                                                              9.7                                                                              9.7                                                                              9.7                                                                              9.0                                Ketone                                                                        Zine Octanoate (18% Zinc)                                                                  0.03                                                                             0.03                                                                             0.03                                                                             0.04                                                                             0.025                                                                            0.03                                                                             0.05                                                                             0.03                                                                             0.03                                                                             0.03                                                                             0.02                               __________________________________________________________________________    Compositions for second pass impregnation                                                        L           M  N                                           __________________________________________________________________________    Multifunctional epoxy,                                                                           90.3                                                       Araldite 8213 (75% solids)                                                    Difunctional Epoxy             87.3                                           Araldite 9302 (72% solids)                                                    Diglycidyl ether of               47.9                                        tetrabromobisphenal A,                                                        Araldite 8011 (72% solids)                                                    Cresol Novolac, Araldite 1280     39.7                                        Hardener, Dicyandiamide        3.0                                                                              2.7                                         Solvent, Methyl Ethyl Ketone                                                                     9.7         9.7                                                                              9.7                                         Catalyst, 2-Methyl Imidazole                                                                     0.13        0.2                                                                              0.25                                        __________________________________________________________________________

An additional application of the subject invention allows slightlymodified formulations of the same resin to be used for first and secondpass impregnating to gain other beneficial effects.

A base epoxy can be modified with the addition of small amounts ofdiluents known to the glass cloth.

Examples are:

1. Silanes and the like for improved adhesion between glass and resin.

2. Ion scavengers, e.g., benzotriazol for Cu, to prevent CAF formation.

3. Tougheners, e.g. for reduction of drill fracture and fiber plateback;e.g., PSF-TB from National Starch and Chemical Co., BIS-AF-PSF, alsofrom National Starch and Chemical Co.

4. Surfactants and/or wetting agents, e.g. BYK A525 or BYK A555 or BYKW980, all from BYK Chemie, USA, for better resin wetting of the glasscloth and reduced laminate striations.

However, many of the additives which give the specific benefits aboveare also known to have various detrimental effects when added to thebulk composite resin. These effects include reduced glass transitiontemperature, degradation of electrical properties and increased bulkmoisture absorption. Thus, addition of these materials only in theamounts and location where they are needed is advantageous.

Additionally, the same can be done for the second pass epoxy which maybe adhered to copper (or some other metal), or organic laminate of thesame or different composition:

1. Adhesion promoters such as silane block copolymers;

2. Copper adhesion promoters, e.g. N-(2aminoethyl-3-amino propyl)trimethoxy silane or trimethoxysilylpropyl-diethylenetriami

3. Reactive coupling agents to improve adhesion to whatever the adjacentmaterial will be, e.g., trimethoxystyryl silane.

The invention has been described as used with woven glass fiber;however, non-woven fiberglass fabric can also be used even though theinterstices are not as pronounced as in woven fabric.

The invention, while particularly useful for forming chip carriers wherethe test requirements are stringent, nevertheless, can be used informing circuit boards if the testing requirements and use conditionindicate this additional cost is justified.

The invention is not limited to two passes or two materials. Thisconcept is extendable to multiple passes to allow a gradual transitionin properties to optimize performance as required.

Accordingly, the preferred embodiments of the present invention havebeen described. With the foregoing description in mind, however, it isunderstood that this description is made only by way of example, thatthe invention is not limited to the particular embodiments describedherein, and that various rearrangements, modifications, andsubstitutions may be implemented without departing from the true spiritof the invention as hereinafter claimed.

What is claimed is:
 1. A resin impregnated layer of cloth comprising,asheet of cloth having fibers and interstices between the fibers, a firstcoating of a first selected thermosetting resin surrounding said fibers,and filling some, but not all, of said interstices, a second coating ofa second selected thermosetting resin different from said firstthermosetting resin disposed over said first coating and with said firstcoating essentially filling all of said interstices unfilled by saidfirst coating of resin, said first coating being cured sufficientlybeyond B stage cure so that it has not dissolved in the uncured resin ofthe second coating, said second coating being B stage cured, atransition zone between said first and second coatings that is smooth,substantially continuous with crosslinking between said first and secondcoatings providing an essentially continuous polymer of two layers; andsaid first coating having better adhesion to cloth fibers than saidsecond coating, and said second coating having better adhesion to metalthan said first coating.
 2. The invention as defined in claim 1 whereinthe cloth is selected from fiberglass, s-glass, quartz and organicfibers.
 3. The invention as defined in claim 2 wherein the cloth isfiberglass.
 4. The invention as defined in claim 1 wherein the cloth iswoven cloth.
 5. The invention as defined in claim 1 wherein said firstresin is at least 60% cured.
 6. The invention as defined in claim 1wherein each said resin for said first pass is selected from epoxies andbismaleimide triazine, or mixtures thereof, and polyimides, cyanateesters, and mixtures of cyanate esters and epoxies, and said resin forsaid second pass is an epoxy or a mixture of epoxies.
 7. The inventionas defined in claim 6 wherein the first resin is a mixture of epoxy andbismaleimide triazine.
 8. The invention as defined in claim 6 whereinsaid second resin is an epoxy.
 9. The invention as defined in claim 1wherein said first layer is cured to at least 60% of total cure.